Field of the Invention
The present invention relates to the structure of a Josephson junction, comprising a non-superconducting structure sandwiched between two superconducting layers. The structure of the invention makes it easier to set up a Josephson junction in making it possible to derive advantage from greater coherence lengths (the coherence length being the length over which the Josephson effect is exerted) with the so-called high critical temperature superconducting (HCTS) materials.
Discussion of the Background
The Josephson effect is an effect related to superconductors, according to which a superconductive current or supercurrent can cross a barrier between two superconductors. If the barrier is insulating or semiconducting, its thickness should be in the range of the coherence length. If the barrier is a normal metal, its maximum thickness depends on the coherent length in the superconductor and on the mean free path of electrons in the metal.
A Josephson junction is constituted by a barrier of insulating material between two layers of superconductors: if the layer of normally conductive material is fine enough, and has a thickness at most equal to the sum of the coherence lengths of the two layers of superconducting materials, then a supercurrent flows through this layer of insulator material.
In fact, it is very difficult to make a Josephson junction for, depending on the nature of the non-superconducting material (metal, semiconductor or insulator,) the barrier layer has a thickness of 0.3 to 0.5 nm. It is very difficult to obtain a barrier such as this having a thickness that is even, homogeneous and without short-circuits.
When the superconductors are copper, barium and rare earth oxides, of the group known as YBaCuO, the invention provides an improvement to the making of a Josephson junction by proposing the use of the greatest coherent length of high critical temperature superconductors parallel to the saturated CuO planes, in the YBaCuO crystal. Consequently, if at least one of the two layers of superconductors, which are in contact with a barrier to form a Josephson junction, has its saturated CuO planes perpendicular to the barrier, the coherence length in the barrier is greater. This means that the barrier can be made with a greater thickness (of 1.5 to 2 nm), hence more easily. The invention can be applied to any other group of anisotropic superconductors.
The invention therefore lies in a Josephson junction structure, formed by a non-superconducting barrier layer located between two superconducting layers such that, for at least one of these two said layers, the crystallographic planes of CuO are perpendicular to the barrier layer. It may also be taken to be the case that the longest axis of the cell structure, in the YBaCuO crystal unit cell, is perpendicular to the CuO planes and that, in a Josephson junction according to the invention, the superconducting layers in contact with the barrier are deposited so that the longest axis of their crystal unit cell is parallel to the barrier.